The present invention relates to an automatic tuning apparatus for filter for tuning the characteristic frequency of a filter having a tuning function to a target frequency and a communication instrument including the above. The present invention relates particularly to an automatic tuning apparatus for filter with low electric power consumption performing high-speed automatic tuning and a communication instrument including the above.
For the recent communication instrument (for example, mobile phones), efforts have been devoted on lowering the power consumption, weight saving and small sizing. On various electronic components, it is a general tendency that they are formed as an integrated circuit. On the filters, too, there is a similar tendency. In the past, they were discrete elements outside the integrated circuit. It is often the case that they are now included as gm-C filters in the integrated circuit. Here, the gm-C filter is a filter utilizing a transconductor (gm) and a capacitor (C) within it so that its frequency response can be changed and controlled by voltage or by current.
For example, for mobile phone, a narrow band BPF having about 5 percent band width with respect to its center frequency is needed. It is required to realize a filter having a center frequency of about 0.2 to 0.3 percent accuracy with respect to the target designed value.
In general, dispersions in the manufacturing process of the integrated circuit are ±10-odd percent for resistors, and also ±10-odd percent for capacitors. The characteristic frequency of a filter composed of a resistor and a capacitor produces dispersions of equal to or more than ±20 percent. A gm-C filter included in a communication instrument is constructed to be tunable from outside and tuned in high accuracy at factory.
The characteristic frequency for a band path filter (BPF) means the center frequency f0. For high pass filter (HPF) and low pass filter (LPF), it means cut-off frequency (frequency at the −3 dB point).
In a mobile phone having a gm-C filter within it, if the consumption current of the gm-C filter is large, duration of call of the mobile phone allowed by each battery charge becomes short. If making the battery size large for securing a longer duration of call, weight saving and small sizing of the mobile phone are not realized.
Under such condition, a gm-C filter is required to have as small power consumption as possible. Also, since the tuning time of the gm-C filter influences power consumption, it is important to attempt to lower the power consumption by shortening the tuning time.
A method for tuning the gm-C filter of a first prior art is disclosed in the Japanese Patent Publication No. Hei 9-83294. Method for tuning a gm-C filter of the first prior art is such that, inputting a reference signal to the gm-C filter, by observing the phase difference between the input signal to the filter and the output signal from it, tuning of the filter is processed.
In the gm-C filter (BPF) of the first prior art, the reference signal having a frequency identical to the tuning target value of the center frequency of the filter is inputted. In this case, when the characteristic frequency of the filter coincides with the target frequency, phase difference between filter output signal and filter input signal becomes 0.
If the characteristic frequency of the filter is higher than the target frequency, the phase of the filter output signal lags behind the phase of the input signal. If the characteristic frequency of the filter is lower than the target frequency, the phase of the filter output signal leads ahead the phase of the input signal. To utilize this principle, inputting the reference signal into the filter, the phase difference between the filter output signal and the reference signal is detected.
In the above-mentioned prior art, based on the above-mentioned result, a value of the updown counter is changed. The updown counter outputs its counter value as the control signal to control the filter. The gm-C filter is tuned appropriately according to the counter value.
In the tuning method of the gm-C filter of the first prior art, inputting the reference signal to the filter, the phase of the filter output signal and the phase of the reference signal are compared. The tuning method of the gm-C filter of the first prior art was that, upon the result of the above, judging is made whether the characteristic frequency of the filter is higher or lower than the target value, and then based on the above-mentioned result the tuning of the filter was achieved.
However, when, for example, the dispersion due to the manufacturing process is large, such case that the center frequency (characteristic frequency) of BPF deviates largely from the target value can happen. If the tuning of such BPF is attempted, because of this large deviation of the center frequency of the filter from the frequency of the reference signal, the reference signal is attenuated largely in the filter, and the amplitude of the filter output signal becomes very small. Therefore, the phase comparison between the filter output signal and the reference signal becomes difficult.
When Q value of the BPF is high, even because of a slight deviation of the characteristic frequency from the frequency of the reference signal, the reference signal is attenuated largely in the filter. Therefore, the amplitude of the filter output signal becomes very small.
When applying this method to a 4th order BPF, when the characteristic frequency deviates more than a certain amount to lower-frequency side, the phase of the filter output signal leads as much as 180° or more (delayed phase between 0°-180°) from the target phase value. When the characteristic frequency of the filter deviates more than a certain amount to higher-frequency side, the phase of the filter output signal lags as much as 180° or more (leading phase between 0°-180°) from the target phase value.
Therefore, when comparing the phase of the filter output signal and the phase of the reference signal, it can not be judged that the characteristic frequency deviates to lower frequency side than the target frequency based on the fact that the phase of the filter output signal leads ahead from the target phase value (leading phase between 0°-180°).
The Japanese Patent Publication No. Hei 9-130206 discloses an automatic tuning apparatus for filter of a second prior art having variable Q value of a filter when tuning the filter by the phase comparison. In the automatic tuning apparatus for filter of the second prior art, detecting the level of the filter output signal, and based on this detected result, gain of the signal to be inputted to the filter is controlled. This automatic tuning apparatus for filter is for attempting to tune a filter by carrying out the phase comparison stably even when the Q value of the filter is high.
In the automatic tuning apparatus for filter of the second prior art, the output level of the band filter is detected and the input level of the band filter is changed appropriately. The method performs the filter tuning stably.
In the second prior art, a feedback circuit is newly provided in order to set the band filter input level appropriately. Thereby, in comparison with automatic tuning apparatus for filter of other prior arts, the characteristic frequency tuning apparatus of the second prior art requires longer time for the filter tuning as the feedback circuit requires the time for setting the band filter input level appropriately,
This art further provides a band pass filter for the tuning monitor use in addition to the active filter to be used actually, and the characteristic frequency of this band pass filter is tuned to the target value.
A relative error can exist between the active filter to be used actually and the band pass filter for the tuning monitor use. Therefore, even if the tunings made appropriately by the band filter, there will be a fear that the active filter to be used actually is not tuned accurately.
The present invention purposes to offer an automatic tuning apparatus for filter of small size and power saving type which enables an accurate tuning, and a communication instrument including the above.
The present invention purposes to offer an automatic tuning apparatus for filter which can tune the characteristic frequency of the filter appropriately even for a higher-order filter or a high-Q filter, as well as, to offer a communication instrument including the above.
The present invention purposes to offer an automatic tuning apparatus for filter of low power consumption and performing a high-speed tuning of the filter, and a communication instrument including the above.
The present invention purposes to offer an automatic tuning apparatus for filter with low possibility of erroneous tuning of the filter, and a communication instrument including the above.